This invention relates to a multilevel frequency modulation encoder which may form the modulator or transmitter portion of a modem (modulator-demodulator). Such an encoder may be used to transmit binary data on a data link at a predetermined frequency. While two level devices of this general type are known, this disclosure relates to three or more level encoding.
The type of device covered in this disclosure also has other technical descriptions which may be applicable. Sometimes these devices may be known as phase continuous oscillators, making reference to the fact that there is no discontinuity in the oscillator output when shifting from one frequency to another. A device of this type may also be known as a self-synchronizing oscillator, making reference to the fact that no external source of frequency is required to produce the desired signal output. And finally, the subject of the disclosure may also be referred to as a parallel network oscillator in that there are as many frequency determining elements in the oscillator as there are desired output frequencies and that the frequency determining elements are arranged in somewhat parallel electrical configuration.
To further define the scope of this disclosure, devices of the type described herein may be used as data transmission elements according to a substantial number of different encoding schemes. The method or scheme of encoding binary signals to produce a three frequency output is not material to the subject matter of this disclosure. The literature on this subject is replete with different ternary alphabets and the advocates for each stress the advantages with respect to differing criteria for error-free data transmission. It is sufficient to say that there is at least one acceptable method of transmitting binary data using a three frequency modulated carrier, that method being a return-to-zero modulation scheme in which the center frequency indicates the absence of data transmission, but represents a clock signal, the higher frequency indicates a binary zero or one and the lower frequency indicates the opposite condition from the higher frequency. This particular method of encoding provides clock timing pulses for the receiving unit together with the data information and provides certain advantages in a network system where a plurality of terminals may be communicating with a single or with multiple computers. Naturally, there are extensive possibilities for different encoding alphabets when more than three frequencies are available for modulation.
One appropriate use of a device according to the disclosure herein would be for encoding data for transmission on a cable television system of the type commonly found today in metropolitan and even rural areas. Such a device could make use of the fact that the cable television channels presently in use in cable television systems leave an unoccupied frequency spectrum in each television channel. The unused bandwidth in a particular television channel may be as much as several megahertz. Alternatively, extra channels are available for various purposes, and certain channels could be dedicated to data transmission. Thus, a system according to the Disclosure herein may be designed to have, for example, a center frequency of 68 megahertz, a lower frequency of 64 megahertz, and an upper frequency of 72 megahertz.
It goes without saying, that the needs of our modern society will require greater and greater utilization of data processing and transmission equipment. Naturally, a significant requirement of such equipment will be the minimum expenditure of capital to establish a reliable and effective method of communication of data. In many cities in the United States, and in other places as well, cable television systems are being established for the principal purpose of relaying television signals through high frequency cable transmission systems to individual subscribers. One requirement of almost all such systems has been the availability to subscribers of the means for inserting subscriber signals into the systems which may be received by other subscribers. While this principal purpose has been related to the communication of television pictures, together with an audio signal, data transmission within such a system between subscribers is also possible where the data transmission equipment is compatible with the characteristics of the television channel and with limits and the transmission capabilities of the existing cable system.
Among the factors and requirements of such a system are that time delays and distortions may occur in transmitted data, therefore making desirable a method of data transmission which contains its own internal timing signals. Another requirement of the cable transmission system is a comparatively narrow, or at least confined, bandwidth for transmitted data. Thus, there is imposed a requirement, known to those familiar with communications theory, that the transmitted data signals have no sharp discontinuities or breaks of the type which require a comparatively high bandwidth for transmission. Thus, any transmission into a cable system of the type described would require phase continuous modulation where the transitions from one frequency to another are smooth, both in the literal sense referring to the image which would be displayed on an oscilloscope display of the data transmission and in the sense of communications theory with respect to the bandwidth required to transmit such a signal.
Included in the relevant prior art in this area is the following article which is known to applicant:
Article entitled "Parallel-Network Oscillators", by J. L. Stewart, Proceedings I.R.E., Vol. 43, No. 5, pages 589-595, May 1955.
Applicant is also aware of the following U.S. Patents which show developments in the general field of the present invention but which are believed not to anticipate the present invention: U.S. Pat. Nos. 3,852,681; 3,564,448; 3,458,835 and 3,411,107. These patents describe devices which have a common output and are driven by a collection of gating circuits and delay lines. A somewhat similar circuit to a circuit embodying the present invention is shown in U.S. Pat. No. 3,411,107 which uses monostable oscillators and electrically selected delay lines to generate several output frequencies. The circuit is similar to a circuit embodying the present invention in that it gates multiple delay paths to a common output. The patent, however, shows tapped delay lines and pulse generators not similar to the present invention. Furthermore, the present invention involves an oscillator producing a plurality of different frequencies in response to control signals and does not incorporate the concept of the majority decision circuit shown in the subject Patent. Of all of the above referenced prior art, the I.R.E. article is believed to be the most relevant. That article, however, in discussing two oscillator networks clearly states that while three oscillator networks are possible, their development does not appear practical. The subject article contains substantial theory but does not show the added features of active isolated feedback amplification of the present invention, which features are shown both in the system diagram and in the actual circuit schematics of the embodiment described.